An In Circuit Test (“ICT”) is a method for performing electrical test of Printed Circuit Board Assemblies (“PCBA”), which may also be called electronic assemblies. Typically an electrical test is run using a test fixture with test probes. In some examples, the test probes are spring loaded test probes to make contact with test points on the PCBA. One type of ICT fixture is a vacuum fixture where a vacuum pump is used to close the fixture and move the PCBA into contact with the test probes to enable electrical testing. The process of moving the PCBA in contact with the test probes is often referred to as fixture actuation. When small Printed Circuit Board Assemblies are tested in a vacuum fixture, there is often a relatively small number of test probes and the damping effect of spring loaded probes may be insufficient to prevent the fixture from closing very quickly and the resulting change in loading can occur very quickly which can result in the PCBA being exposed to a high strain rate. Solder joints of PCBA assemblies and especially Lead Free PCBA assemblies are susceptible to solder joint failures when strain rates are excessive. A common practice is to define a limit for the strain rate which a PCBA can experience to reduce a probability of damage.
FIG. 7 depicts test results for a prior art electronic assembly test system showing strain rate and a strain rate limit. Strain gauge rosettes are devices placed on a PCBA to measure strain and often take the form of three stain gauges positioned to measure strain in multiple dimensions. One orientation is to have one strain gauge at 0 degrees, one at 45 degrees and one at 90 degrees. The test results of FIG. 7 are for two rosette locations on a PCBA. The PCBA is a relatively small PCBA. The vertical axis is strain, which is a dimensionless unit and represented with an “e.” The units for the vertical axis are μe or micro-e. The horizontal axis depicts strain rate in μe/s or micro-e per second. An end user will usually define a set of safe design limits where the maximum strain and strain rates are identified such that damage due to strain or strain rate is typically prevented. FIG. 7 shows an example of safe limits.
During fixture actuation, the PCBA flexes and may cause damage, including damage to solder joints. The points on the test results show measurements from two rosettes. Measurements were made at a particular sample rate during fixture actuation and release and the data points depict the measurements at the rosette 2 location and the rosette 3 location. In the test results depicted in FIG. 7, strain is recorded at a sample rate of about 1 kilo hertz and the strain rate is calculated from the sample data. Max and min principal strains are calculated from the three strain gauges in a rosette and provide two values which are acting perpendicular to each other along principal axes.
Typically, a certain amount of vacuum pressure is required to ensure that the vacuum seals in an ICT fixture are compressed properly so that reducing the vacuum pressure beyond a certain point to slow down the fixture actuation is impractical. Note that for rosette 2, that there are measurements close to the design limit, and that for rosette 3, both for the min and max tests, there are data points that exceed the design limit. If the strain rate limit is exceeded, there is a risk that the stress applied to the solder joints may weaken the solder joint or printed circuit board laminate integrity, causing time zero failure, or worst case, intermittent failures that will eventually fail in product use (field fail). Lead free formulations, because of the stiffer nature of the reflowed solder composition, tend to be more susceptible to failure modes (cracks, solder fatigue). Typical solutions for a small PCBA condition may involve using bumper guards or dampening devices. However, when the electronic assembly area size is relatively small, these additional measures are typically not enough.